In many industrial applications it is necessary that conduits or ducts be provided for interconnection of various types of apparatus which require the routing of hot gaseous fluids of many types from one component to another. In most of these environments, the hot gaseous fluids exhibit not only corrosive characteristics which will adversely affect conventional metallic conduits, but are also caused to flow at relatively high velocities. The relatively high gas flow velocity frequently encountered results in further adverse effects of either frictional wearing and erosion of the interior of metallic ducts or other types of duct work including but not limited to the increased likelihood of chemical corrosion. The relatively high temperatures also have a substantial adverse effect on the metal conduit such as decreasing structural strength and presenting a safety hazard to workers who may need to be in close proximity.
One technique heretofore utilized to meet the requirements of this highly adverse operational environment has been to line metallic or steel ducts with a castable refractory material. A disadvantage of the ducts having the castable refractory liner is that such refractories are not thermally efficient and as a consequence, the metal ducts which are lined with such materials must be made so as to develop greater structural strength to support the weight of the castable refractory that is required in most cases to meet the operational requirements.
There has also been an attempt to form ducts capable of withstanding the operational requirements of transmission of hot gaseous fluids through lining of metal conduits with a soft ceramic fiber material. The soft ceramic fiber material, as its name implies, does not exhibit the desired resistance characteristics to erosion through the relatively high gas velocities that are encountered. In an effort to meet the erosion effect, there have been attempts to also coat the interior surface of these liners with a layer of suitable material to attempt to rigidify the interior surface layers. These attempts have also not proven to be sufficiently successful as the rigidifying surface material will eventually crack and peel off and expose soft ceramic fibers to the point where the liners will erode and eventually become unusable, thus requiring replacement.
Another disadvantage of either the castable refractory liners or the soft ceramic fiber lining is the technique of obtaining the attachment or positioning of the liner within the metal conduit. The usual techniques require first securing of anchors to the interior of the metal duct. The refractory material is then either mechanically secured or it is formed directly onto those anchors such as by spraying or molding. Similarly, the soft ceramic fiber linings have been applied to the interior of the metal conduits by use of anchors which are first secured to the interior of the metal conduit.
Another attempt to meet the problems has been the formation of a vacuum cast sleeve or liner which is then subsequently assembled with a metal duct. This technique is not particularly advantageous in that it requires assembly at the operational site where the duct will be utilized. This technique increases the cost of installation.